The molecular basis of photoexcitation in the ventral photoreceptor of the horseshoe crab Limulus is under investigation using intracellular microelectrode recording, voltage-clamp techniques, and pressure injection of materials into the photoreceptor cell. The role of calcium in initiating light induced conductance changes is being examined. Quantal bumps are discrete depolarizations each due to a single photon absorption that can be observed in this preparation. The statistical properties of quantal bumps, especially their latency dispersion is being examined as an expression of the underlying molecular events in photoexcitation. Quantal bump latency depends upon intracellular and extracellular calcium concentration. The detailed relationship between the quantal bump latency distribution and the concentration of extracellular calcium, competitive inhibition by magnesium, and the pulse injection of calcium iontophoretically is being examined. The possible role of cyclic neucleotides as an intracellular transmitter substance is being examined by pressure injection of these neucleotides thru a microelectrode.